Batch Generate Previews Not Working
You might be using it wrong, I think you misunderstood how it is supposed to work.

Generate Data previews is not meant to be used for a currently open file, you are supposed to use it on any saved file in your harddrive, not being worked on at the moment.

If you batch generate previews for an open file, Blender will work to generate them then save them to the file. However when you save the open file Blender will write to disk the version it had previously loaded into memory, prior to thumbnail generation, thus not yet containing the generated previews.

It wil then overwrite the the preview file with non generated version, thus effectively erasing the work done.